The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.
Kinetic energy kill mechanisms employed in anti-tank guided missiles (ATGM) are generally produced by impacting the target with a penetrating rod that is carried by a hyper-velocity missile (HVM). In order to achieve the high velocities (generally mach 5 or greater) necessary to produce this kill mechanism, the HVM designer must maximize thrust and minimize drag. These requirements typically dictate a small overall diameter, a sharply tapered nose, a minimum number of appendages (fins, etc.) and a powerful rocket motor producing a large exhaust plume.
ATGMs are typically guided by sensors or data links mounted on either the nose (a terminal homing seeker viewing the target) or the tail (sources and sensors viewing back to the launcher""s fire control system, as in Command-to-Line-of-Sight (CLOS) or Laser Beam Rider (LBR)). It is generally considered impractical to employ a seeker on an anti-tank HVM due to the small diameter and finely tapered nose shape and the severe thermal environment produced on the nose by Mach 5 flight at low altitudes. It is likewise difficult to devise a guidance data link on the tail of the HVM missile because this arrangement causes the sources/sensors to be proximate to the typically large rocket motor exhaust nozzle and necessitates the transmission of the data through much of the large signal-absorbing plume the nozzle produces. Techniques to minimize these effects, such as locating the receiver on pods offset from the missile axis, are often expensive and/or performance-degrading. FIG. 1 shows the direct communication link used in existing LBR guidance systems. In order to avoid communication through the motor plume, the missile-borne, rear-looking light detector must be placed in an offset position relative to the axis of the missile. The signal-to-noise margin associated with this communication link is strongly dependent on the length of the offset and the surface area of the detector, factors that directly degrade missile velocity. The performance-degrading weight and surface area associated with the pod used to house this detector is usually exacerbated by the need to balance the aerodynamic load with another pod mounted on the opposite side of the missile.
A way to by-pass these difficulties is to use an indirect communication path from the launcher to the missile. Electromagnetic radiation (i.e. light) is known to scatter off the naturally occurring particles and molecules in the atmosphere. If, for example, a laser beam of sufficient power is transmitted from the launcher through the air and offset to one side of the flight path of a missile, thus bypassing the plume, light will be scattered laterally from the beam onto the side of the missile. Such scattering effect can be easily observed as the visible column of light from a search-light against the night sky. Appropriate sensors on the side of the missile can receive this signal for guidance purposes. This side-scatter communication approach, therefore, avoids both the aerodynamic and the plume interference difficulties mentioned above.
There are various ways in which the scattering laser beam can be used to impart missile position information to the sensors so that the missile can guide itself along the desired trajectory to the target. The prior art includes three patents (U.S. Pat. Nos. 5,374,009; 5,664,741; 6,138,944) each of which describes the creation of an off-axis guidance link using the existing low pulse rate laser, such as the U.S. Army""s Ground Laser Locator Designator (GLLD), normally used in conjunction with semi-active missile systems such as HELLFIRE. U.S. Pat. Nos. 5,374,009 (Walter E. Miller, Jr. et al.) and 6,138,944 (Wayne L. McCowan et al.) teach a guidance technique known as scatter-rider. The Miller et al. system was devised as a limited-accuracy initial guidance mode for a terminal homing seeker missile. The missile employs side-looking receivers to detect energy indirectly from the laser designator by way of atmospheric scattering. Amplitude differences in the level of received energy associated with these receivers are used by the missile""s processor to keep the missile close enough to the beam axis to permit handoff to the more accurate terminal guidance mode at the appropriate time during the missile flight. The McCowan system was devised as a limited-accuracy, low-cost retrofit to small unguided rockets. Again, the GLLD""s narrow laser beam is transmitted directly on the line of sight to the target. The missile employs both forward and rearward canted side-looking receivers, as illustrated in FIG. 2. Time differences in the temporal waveforms associated with the detected energy are used to determine the approximate lateral direction and distance to the beam. This information allows the missile to turn continuously toward the beam and, thusly, fly roughly down the line of sight to the target. This approach has limited accuracy because the missile sensors cannot determine the direction to the beam center when actually inside the laser beam. As a consequence, it tends to wander off the ideal line-of-sight flight path more than the proven CLOS and LBR guidance systems. However, this limited accuracy was deemed acceptable for a low-cost retrofit of a small unguided rocket but would be inadequate for an anti-tank HVM.
In a variation of scatter-rider, U.S. Pat. No. 5,664,741 (Jimmy R. Duke) adds a circular scanning optical system in front of the same low pulse rate laser to cause the laser beam to describe a circle about the desired flight path. The laser pulses are synchronized with the scan to occur at four fixed locations about the line of sight. The side-looking sensors have multiple narrow fields-of-view so that the direction to each laser pulse can be measured and combined with the others in a scan to calculate missile position relative to the center of the scan circle (the desired flight path). This approach overcomes scatter-rider""s loss of accuracy near the line of sight, but is still insufficiently accurate for long range precision guidance applications due to the practical limits of the segmented receiver""s optical system. Increasing the accuracy of the approach would require a greater number of smaller segments, at the cost of reducing the guidance link""s signal-to-noise margin. In addition, the approach incurs a loss of guidance data rate by requiring multiple pulses (typically 4) to be used for each position calculation. The semi-active target designation lasers operate at 10 to 20 pulses per second, providing a guidance data rate of only 5 Hz, inadequate for hypervelocity flight.
It is the object of this invention to provide a guidance system that combines the advantages of side-scatter communications described above with full accuracy and high data rate for a kinetic energy ATGM missile.
In accordance with this invention, a beamrider guidance link is provided in which a pulsed laser projects into the guidance field a beam that is spatially encoded with azimuth and elevation scans of pre-determined angles. This encoded beam is indirectly relayed to side-looking missile-borne receivers by way of scattered radiation effected by atmospheric particles. Multiple optical receivers mounted on the side of the missile, each receiver having a different field-of-view (FOV) from its adjacent receivers, receive light from the transmitting laser that is thusly scattered by atmospheric particles. In response to the received scattered radiation, the missile""s signal processor calculates the missile""s position within the guidance field by determining the precise time at which the detection of scattered beam shifts from one receiver to an adjacent receiver. It then generates steering commands necessary to remain in or near the center of the guidance field, which center is normally coaxial with the target line-of-sight (LOS).